For Auto-Tune Evo, Auto-Tune EFX, AVOX 2, Harmony Engine, AVOX, Auto-Tune 5, and Auto-Tune 4:

How do I Authorize new software?
http://portal.knowledgebase.net/article.asp?article=182497&p=5764

How do I Authorize using an iLok?
http://www.antarestech.com/downloads/iLok_Authorization_Guide.pdf

How to Authorize an iLok if your music computer is not connected to the internet
http://portal.knowledgebase.net/article.asp?article=134818&p=5764


For Tube, Filter, Mic Modeler, Kantos, Infinity, Auto-Tune 3 and earlier:

How to Reauthorize a plug-in
http://portal.knowledgebase.net/article.asp?article=107813&p=5764

How to Authorize using Challenge Response
http://portal.knowledgebase.net/article.asp?article=137204&p=5764